Hi MarcoOriginally Posted by Marco
Interesting idea, and one that could work. This is far from an urgent project though (just floating some ideas about, really), so I won't be rushing into anything. As usual, when I've decided to go ahead, extensive research will be undertaken, and the processing of as much relevant info as possible will be carried out before a choice is made.
Yes, so they say, and in certain circumstances that may be true.
However, I've used umpteen different passives in my system with the Copper amp, which is even more sensitive than a Quad 306, and high-efficiency speakers (95db Tannoys), and where on paper a passive should thrive, yet (aside from LDRs), when used the soundstage collapses and they *always* sound soft/veiled/'padded out'/lacking in 'drive' and dynamics (even receiving a 2v digital signal), *in comparison* [that bit is crucial] with the best active designs.
Therefore, given the above and the type of system I've tried passives in, I'm struggling to see what the 'compatibility issues' would be?
Don't get me wrong, the sound isn't dreadful or anything like that, with passives, just not up to the high standards I'm used to from top-notch active designs. You have to remember the benchmark I've got as an active preamp...
My modded Croft, in today's market, would likely retail for around £8-10k - it's a very capable, highly-optimised design, which allows serious insight into recordings, and a truly wide-open window onto the music, so that's what any passives are up against.
Interesting though, that LDRs, some of them despite not sounding anywhere near as good as the Croft, don't suffer from a lack of said 'drive', and consequently don't sound soft, so there must be something in their design that negates this.
They're powered by the mains supply, so perhaps that's part of it?
Marco.
LDR's are capable when properly powered to present music exactly as it was recorded. They neither add or subtract from presenting every bit of detail, dynamics that the recording has preserved.
Properly powering them has been my passion and interest for over 13 years. The type of LDR firstly is the encapsulated type NSL32SR3 . The SR3 has the best properties for audio as well as capability of highest resistance range.
The circuit formed on the signal side is a passive L pad, however with a distinct difference, to simply being just variable resistance ( achieved by more or less light intensity ). The difference is the enormous hold or grip which LDR's have on the entire grounding of all audio equipment, that it attaches to. The benefit is a black silence in a unbalanced audio system, that even balanced systems cannot achieve. To make this occur the LDR anodes needs to receive ideally 8 volts or more and have its cathodes not directly grounding, rather far better forming active current drive for the circuitry.
If we think of the most outstanding amplifier circuits that preserve bandwidth such as the cascode and common gate, and we apply those forms to what is ostensibly a DC circuit, we find we suddenly are confronted with added audio properties
occurring on the signal side. Some might say DC circuitry does not need high bandwidth as it is form, is the exact opposite, however if we read manuals such as Designing with Field Effect Transistors 2nd Edition reference is indeed made to using cascodes in DC circuitry. I regularly inform audio forums who muster all sorts of opposition to LDR's, that they simply relay how well or how poorly they are powered, power them well if we speak in positive terms and that is what you hear.
Comparison then to other passives that do not have this grounding grip as we are calling it , has LDR's in a area all of their own. Other passives use ideally or represent approximately 15 k of resistance at mid volume in usually equal halves as a potentiometer, variations are those with log curves meaning there is usually a taper in the volume curve . The benefit with LDR's is they emphatically do NOT have to follow potentiometer set curves and forced restrictions.
We are thus free with LDR's to have total control over where the shunt pair becomes active vs the series pair. Another is that we can have total silence at zero volume, simply by arranging a voltage reference pushing appropriately against the series cathodes. Which begins to ask, what is ideal resistance of the potentiometer used in LDR's and what is its best positioning. In my circuits I can proudly say there are NO resistors on the main board - none at all ! Rather I have solved all need of resistors, with semiconductors. Most of the semiconductors used therefore are very close internally to exactly half the potentiometer resistance value, so the potentiometer used for attenuation is just seen as yet another semiconductor resistance value and in effect semi disappears as a device. The potentiometer is placed between the series cathodes volume UP, and shunt cathodes volume down. When measured we find some really good impedance characteristics for matching to source components and to power amps. Typically 1k3 on the shunt pair at half volume and 2k to the series pair. If we compare this to a stepped attenuator or a potentiometer passive it is losing badly in terms of connecting the source component and power amp, as it has much higher resistance in order to function. LDR's therefore connect not only all your grounding paths with grip, but also connect to the source component and power amp in near perfect relationship.
I hope that helps to answer some of the mysteries. Please let me know if I can help further.
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